Sol-gel synthesis of Eu³⁺ doped silica-gold nanorod composites with tunable optical properties

Gold nanorods (AuNRs) suspension at various concentrations was added into the sol-gel process to engineer nanostructured europium-doped silica host matrices as light-emitting composites. For this purpose, the samples were prepared following two different routes depending on the chemicals used as dopant and catalyst: (a) Eu(NO₃)₃·5H₂O and HNO₃, and (b) EuCl·6H₂O and HCl. In any case, samples adding various concentrations of AuNRs suspension were prepared. The structural characterization of the samples was through STEM, backscattered electrons (BSE), and EDS analysis. Additionally, their optical properties were evaluated by PL spectroscopy and CIE colorimetry. The results confirmed that (a) methodology produced samples with AuNRs embedded and randomly distributed in the samples. However, these features were not observed in the samples obtained through (b) due to AuNRs dissolution in HCl media. Regarding the optical properties, the analysis of the relative intensity ratio ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ suggested that Eu³⁺ ions occupy non-centrosymmetric sites in (a) host matrices and centrosymmetric sites in (b). Hence, the increase of AuNRs suspension when fabricating (a) host matrices produced remarkable color changes in the luminescence of the samples towards the reddish-orange region. Meanwhile, the dissolution of AuNRs in (b) minimized the localized surface plasmon resonance (LSPR) effects on the Eu³⁺ luminescence. These findings revealed that the evaluation and selection of chemicals are critical factors when engineering these materials for more efficient coupling between the LSPR and Eu³⁺ luminescence.